As a refrigerant for a refrigeration cycle, a CFC substitute refrigerant (R134a) is widely used. In the refrigeration cycle driven by this CFC substitute, an evaporator constituting a kind of an external heat exchanger is arranged downstream of an expansion valve, and a refrigerant, reduced in pressure by the expansion valve, flows into the evaporator. In the evaporator, the refrigerant is evaporated (gasified) by exchanging heat with the air and, after absorbing heat from the surrounding air, changes the air into cool air. The evaporator comprises one or a plurality of rows, arranged along the thickness, each including one or a plurality of unit cores each having a multiplicity of juxtaposed heat transmission tubes in which the refrigerant flows, a first tank connected to one opening of the heat transmission tubes and having a refrigerant supply path or a refrigerant discharge path and a second tank connected to the other opening of the heat transmission tubes and having a refrigerant supply path or a refrigerant discharge path.
In view of the fact that the refrigerant is in a gas-liquid double phase in the first and second tanks, the shape and size (diameter, length) thereof has a great effect on the distribution characteristic of the refrigerant to the heat transmission tubes. In a case, for example, in which the first tank having the refrigerant supply path is connected to the lower end opening of the heat transmission tubes and the second tank having the refrigerant discharge path is connected to the upper end opening, while the refrigerant is moved upward in the heat transmission tubes, the liquid refrigerant and the gas refrigerant are dispersed in the portion near the inlet of the first tank, while the liquid and gas refrigerants begin to be separated from each other at the intermediate portion. At the portion far from the inlet, the liquid refrigerant and the gas refrigerant are separated from each other, and the liquid refrigerant is stored in the first tank by the force of inertia so that CFC (chlorofluorocarbon) refrigerant, containing a large amount of liquid refrigerant, moves upward along the heat transmission tubes.
As described above, the ratio between the liquid refrigerant and the gas refrigerant is varied between the portions near and far from the inlet of the first tank in the transverse direction of the unit core. The liquid refrigerant contributes to the cooling operation, while the gas refrigerant does not substantially contribute to the cooling. Thus, the variation of the cooling temperature (uneven temperature distribution) occurs between the portions near to the inlet and far side from the inlet. The lack of uniformity of the temperature distribution tends to be significant in the time of a low refrigerant flow rate where the gas-liquid separation is promoted.
In the conventional evaporator (Japanese Unexamined Patent Publication No. 2001-074388), in contrast, a throttle is arranged, at a portion far from the refrigerant inlet/outlet in the tank for inflow and outflow of the CFC refrigerant, to control the flow of the liquid refrigerant.
In the prior art, a particular longitudinal portion (range) of the tank where a throttle is arranged cannot be easily determined, and it is difficult to accommodate the transverse size change of the core. Also, the throttle arrangement described above, though effective for the evaporator operated with the CFC refrigerant, is not necessarily effective for a closely-watched evaporator operated with a carbon dioxide gas refrigerant. Specifically, the operating pressure of the carbon dioxide gas refrigerant in the evaporator reaches as high as about ten times that of the CFC refrigerant and, in order to accommodate this high pressure, the tank plate thickness is required to be increased or the pressure-receiving area in the tank is required to be reduced (the tank diameter is required to be reduced). It is still unknown how the throttle in what shape is suitable to be arranged in the tank thick and small in inner diameter.
The throttle arranged blocks the refrigerant flow and generates a pressure loss. Further, a carbon dioxide gas refrigerant, as compared with the CFC refrigerant, has different physical values. The gas-liquid density difference of the carbon dioxide gas, for example, is about 1/80 and considerably different from that of the CFC refrigerant of about 1/8.5. This gas-liquid density difference is related to the gas-liquid separability.